I. Technical Field
The present invention relates to a welding wire and welding method.
II. Description of the Related Art
Safety is strongly required regarding the welded portions of structures using high grade steels such as 9% Ni steel and stainless steel intended for low temperature or intended for atomic energy, chemical plants, etc., and it is essential to form defect-free welded metals with excellent toughness and ductility. In order to obtain high quality welded joints in such structures, welding in an inert gas (argon, helium or a mixture gas thereof) atmosphere is indispensable. When welding in an inert gas atmosphere, only non-consumable electrode tungsten inert gas (TIG) arc welding of low production efficiency is available presently. TIG welding has been carried out for as long as 30 or more years at the expense of productivity.
On the other hand, although arc welding of excellent production efficiency, such as consumable electrode (welding wire) MIG or MAG arc welding has been employed widely in a production site, it is considered impossible to apply this welding method to an inert gas atmosphere. The reason is that the concentration point (cathode spot) of a local arc current formed in the surface oxide of a material to be welded (base material) moves around randomly and violently in search of an oxide, and the arc becomes unstable leading to the generation of welding defects such as meandering beads, blow holes and undercuts, thereby not forming a good welded portion.
In MIG arc welding, in order to solve the instability of the arc in the case where the welding is carried out in an inert gas atmosphere, it is proposed to add roughly 0.5 to 5% of an active gas such as oxygen gas or carbon dioxide gas to an inert gas atmosphere (shield gas) and form an oxide on the weld metal (weld pool) surface to restrain the behavior of the cathode spot. However, although the addition of an active gas certainly is effective in stabilizing on the arc, 200 ppm or more of oxygen is contained in a weld metal, and thus poses a problem in that the formation of fine oxide inclusions lowers the ductility and toughness of the welded metal site.
Therefore, a welding method in an inert gas atmosphere that can achieve both high productivity and high quality at the same time is desirable.